The Victorian government has mandated COVID-19 vaccines for all construction workers while immediately banning them from crossing the regional-metro divide.

Victorian construction workers have a week to get vaccinated and keep their jobs, as the state tries to slow the spread of COVID-19 in the industry and protect regional areas.

The Victorian government has moved to mandate vaccines for construction workers across the state after the virus spread into Ballarat, forcing the regional city into lockdown.

A further three cases were reported in the Victorian gold-rush town on Thursday after two contacts of an infected construction worker tested positive.

It brings the cluster to six, plus an unlinked case that has been infectious in the community since last Tuesday.

In response to the outbreak, all Victorian construction site workers will be required to show evidence to their employer they have had a first dose of the COVID-19 vaccine by 11.59pm on September 23.

Limited medical exemptions and proof-of-booking exceptions will apply, in keeping with the vaccine mandate for aged care workers.

Premier Daniel Andrews said the decision was backed by health advice and designed to keep the construction industry open at its 25 per cent workforce cap.

"If they want to work and be part of that 25 per cent, they need to be vaccinated with one dose by midnight next Thursday night," he told reporters.

"If they're not, they won't be able to come on site. That's keeping them open. The other thing would be to close them down to zero."

With about 13 per cent of active cases in the state linked to construction sites, the industry was earlier this week warned it risked losing its authorised worker status amid the launch of an enforcement and vaccination blitz.

Victorian state construction union secretary John Setka on Tuesday said the CFMEU was "pro-vaccine" but did not support mandatory workplace vaccination.

In further changes from 11.59pm this Friday, construction site workers are banned from crossing the metro-regional boundary for work, tea rooms must shut, and food and drink can no longer be consumed indoors.

Mr Andrews said he wanted to avoid a repeat of the situation in Ballarat, with no more regional Victorian towns needing to revert to lockdown because of industry-related seeding events.

"If we see all this construction movement, we will see more cases. That will mean we have no choice but to lock down bigger and larger parts of regional Victoria," he said.

The ban on construction work travel between regional and metro areas will go both ways and also apply to the sourcing of supplies.

Construction workers unable to do their jobs because of the travel boundary ban will be eligible for income support.

"We'll do whatever we can to support those people," Mr Andrews said.